Micro-crack detection is important in manufacturing of silicon (Si) wafer solar cells and modules, because micro-cracks may reduce the mechanical strength of Si wafers and lead to wafer breakage in the production line. Not only does the breakage reduce the manufacturing yield, thereby increasing the production cost per solar cell, but also cleaning wafer fragments can be very costly if a cell breaks at a critical step (e.g. etching bath, plasma-enhanced chemical vapor deposition (PECVD) chamber, etc.). Micro-cracks in the wafers could also develop into a shunt and reduce the solar cell efficiency, even if those wafers did not break in the manufacturing process. In addition, if solar cells with micro-cracks are encapsulated into a module, these micro-cracks could grow larger during module operation in the field, causing problems in the long-term reliability of a photovoltaic module.
The impact of smaller micro-cracks becomes more pronounced when Si wafers become thinner than the current standard thickness of 180 microns. Utilizing thinner wafers has significant cost benefits. Wafer thickness reduction is an effective way to achieve dollar-per-watt variable cost reduction, and it is also one of the highest-impact single factors to reduce capital expenditure (capex) for photovoltaic (PV) module factories. However, high breakage rates of thinner wafers in the production line, as well as relatively low polycrystalline silicon (poly-Si) price, discourage thickness reduction in today's PV industry. The impact of micro-cracks is especially significant when they intersect a side of the wafer because such cracks are particularly likely to propagate when stress is applied to the wafer or when the wafer is subject to strain.
Two exemplary methods of crack detection that have been used in silicon wafer production are: luminescence imaging, and optical imaging using near-infrared (NIR) or electromagnetic radiation (also herein referred to as light) of other wavelengths. Luminescence imaging, including photoluminescence and electroluminescence, relies on the physical principle that increased non-radiative recombination at the crack interface results in a dark feature the luminescence image. This technique may work well for monocrystalline silicon (mono-Si) wafers, but not for multicrystalline silicon (multi-Si) wafers due to severe interference by recombination-active features (e.g., grain boundary, dislocation, metal contact region, etc.). Optical imaging, using NIR or light of other wavelengths, is better suited for crack detection in multi-Si wafers. A common technique of optical imaging (e.g., optical NIR imaging) is rear-illuminated transmission imaging. A main concern for rear-illuminated transmission imaging is that the technique cannot be applied for finished solar cells with full area rear contacts. A second problem for rear-illuminated transmission imaging is that the sensitivity for small micro-cracks is reduced as the crack width becomes smaller than the pixel size. For example, an apparatus that uses a side-coupled NIR laser illumination, and scans the wafer with a linescan camera synchronized and collinear with the light beam, may fail to detect cracks that are aligned with the light beam. Other techniques that have been used for crack detection, such as scanning acoustic microscopy and ultrasonic thermography, have low throughput in detecting micro-cracks.